Future regulations on emissions require significantly reduced production of pollutants from an internal combustion engine. Engine manufacturers have responded by developing a number of systems directed at reducing pollutants. Regulated pollutants include unburned hydrocarbons, carbon monoxide, oxides of nitrogen (NOx), and particulates. Exhaust gas recirculation (EGR) is a system to reduce the formation of NOx.
In an exhaust gas recirculation system, exhaust gas replaces a portion of fresh air in an inlet air stream. Substituting exhaust gas for fresh air in the inlet air stream reduces a mass of excess oxygen in the inlet air stream. Excess oxygen is the mass of oxygen above that mass needed to create a stoichiometric mixture of air and fuel. Reducing excess oxygen in the combustion chamber slows a combustion process. A peak gas temperature in the combustion chamber decreases with the slower combustion process. High peak gas temperatures along with the excess oxygen in the combustion chamber are generally associated with the formation of NOx.
Some exhaust gas control systems further reduce NOx by retarding injection of a fuel into the combustion chamber. Retarding injection reduces a mixing time where the fuel and air are able to mix prior to the combustion process. The reduced mixing time in effects limits the excess oxygen in close proximity to the fuel. With less excess oxygen available to combust the fuel, the peak gas temperature in the combustion chamber decreases. The combination of reduced excess oxygen and lower peak gas temperature reduces the production of NOx. However, the retarded fuel injection increases a production of particulates from the engine. Generally, particulates form during incomplete combustion. Incomplete combustion of the fuel may also reduce the efficiency of the engine.
The increased particulates tend to cause problems in an engine having an EGR system. Many EGR systems include a heat exchanger to reduce the temperature of exhaust gas being recirculated. Cooling the exhaust gas further reduces the production of NOx by further lowering the peak gas temperature. To maintain the effectiveness of the heat exchanger, the particulates must be removed from the exhaust gas prior to entering the heat exchanger. Particulates entering the heat exchanger will foul the heat exchanger. The fouling of the heat exchanger will result in less effective cooling. To maintain the proper cooling, the manufacturer may oversize the heat exchanger.
To reduce the fouling of the heat exchangers, many manufacturers include a particulate trap ahead of the heat exchanger. Particulate traps also are subject to fouling. Fouling may be avoided by creating larger passages. However, larger passages will likely lead to blockages forming in the heat exchanger. Many particulate traps increase a face area perpendicular to a recirculation flow. The increased area allows for smaller passages while retaining a flow area equivalent to a flow area in an upstream duct. To reduce maintenance, manufacturers may increase the face area in anticipation of a portion of the smaller passages being blocked. The expansion and contraction of the recirculation flow passing through the particulate trap creates a loss in pressure. For an exhaust gas recirculation system to work in a high load condition, pressure losses from the exhaust manifold through a recirculation conduit must be reduced.
The present invention is directed at overcoming one or more of the problems set forth above.